iPhone 4 iPhone 4 data usage? sent and received?

before I begin, Sorry for sounding like a bit of an idiot, this is my first iPhone and first contract with internet usage.
When I go to settings > general > usage and look at the data usage, im told how much has been sent, and how much has been received, I have 1GB data usage a month, and the received usage is always higher than the sent.
What do I need to keep an eye on to make sure I dont go over? Sent? Received? both?
Again, sorry for being a noob.
Thanks.